In this type of method, the concentration of the gas is determined from its optical absorption characteristics by measuring the intensity of radiation in various wavelength bands transmitted through the gas mixture.
This technique is very sensitive to temperature and gives variable results when the ambient temperature changes, on the one hand because of the resulting modification of the characteristics of the components making up the analyser, in particular the semiconductor-based optical components, and on the other hand because of the variation in the absorption spectrum of the gas to be analysed as a function of temperature, this variation being due in particular to a variation in its density.
In order to compensate for the measurement errors caused by the variation in the sensitivity of the optical components as a function of temperature, it is known to introduce, into the analyser, a thermistor, the temperature coefficient of which is chosen so as to be equal to that of the photoreceiver optical component, associated with an adjusting resistor in order to set the information provided by the analyser to zero in the absence of the gas to be analysed. Thus, the influence of temperature on the photoreceiver is neutralized.
Moreover, in order to neutralize the influence of temperature on the absorption spectrum, a diode is used whose temperature dependence is similar to that of the absorption spectrum of the gas to be analysed. This diode controls a calibrating voltage supplied to the radiation source so as to compensate for the variation in gas density.
This technique requires a prior step of calibrating the analyser, during which an optical analysis cell is filled with a gas at a known concentration N and the calibrating signal is adjusted so that the information provided corresponds to this concentration value.
This technique has a number of drawbacks, especially because the calibration operation must be carried out again when the ambient temperature changes.
Furthermore, these operations are not always able to be accomplished in that it is not possible to determine, using an uncalibrated device, the absence or presence of the gas to be analysed in the environment.
Moreover, the compensating elements, such as the thermistors or diodes, only allow an approximate compensation to be made for the temperature dependence of the sensitivity of the photoreceiver and of the absorption spectrum of the gas to be analysed.
Finally, this type of method does not make it possible to compensate for the variation in the characteristics of the components other than the photoreceiver.
The object of the invention is to overcome the aforementioned drawbacks.